Environment lighting(Blender internal)
Tip: Start from a pitch-black scene:


Set all World colors (Horizon, Zenith, Ambient) to Black (hex 000000)
Turn off all other forms of Lighting (ambient, environment, sky, indirect)
Turn off all lamps (or move them to another 3d-viewport) layer


So that your scene now becomes totally blacked out. 

Then start adding various light sources from the TOP -> Down:

Global lighting first:


World (sky, horizon, zenith)
Ambient occlusion
Environment lighting
Indirect lighting 


then add Local light sources:


Lamps (sun, area, hemi, spot, point)
Turn on any material emission (which bring light into the scene)


En voila... now you know and understand which light source effects what in kind of lighting effect inside your lovely scene.